The present invention relates to a method and a system for measuring heart rate variability (HRV).
The study of HRV has been in use for many years as part of clinical, prognostic work; there are international guidelines for evaluating conventional HRV parameters. The conventional parameters are partly frequency domain parameters (power spectra), and partly time domain parameters (various RMS estimates). These methods, though in general successful, are not always conclusive.
Over the last few years, new methods of analyzing Rxe2x80x94R intervals of PQRS plots representative of the human heartbeat wave have appeared, all of them showing improved diagnostic and prognostic performance. It has been shown that so-called scale dependent methods outperform scale-independent measure with respect to separating healthy subjects from patients suffering from certain cardiac dysfunctions. But in clinical practice, it is of interest to examine whether, within a group of heart patients, one can extract a subgroup of patents who are at risk, e.g., with respect to sudden cardiac death, rather than to verify the presumably known fact that they do not belong to a group of healthy subjects. It has been shown that, while scale-dependent methods worked in the former case, one had to use scale-independent measures in the latter case.
In practical medicine, recurrence plots of the Rxe2x80x94R intervals are used for diagnostic purposes by visual inspection. Since the density of the points is ignored in a visual presentation of the plot, sometimes similar patterns are found for recordings with different HRV""s. Although some attempts have been made to include the density of points, this procedure is performed manually and is thus dependent on the performer""s skins. Hence, crucial information about the topology of the recurrence plot might be lost.
FIG. 1 illustrates an ECO signal wave. The electro-physiological features of the heart are generally measured by an electrocardiograph, and the electro-physiological recording of the heart function is known as the ECG or EKG. The six features P, Q, R, S, T, U (FIG. 1) describe the sequence of two cycles wherein the R potential is the highest peak. It is therefore easy to distinguish the other five features and the R-peak of the next sequence. The Rxe2x80x94R distance is measured in milliseconds and represents the inverse heart rate (HR). The HR is normally not constant, but continually oscillates around its mean level. These short-term cyclic changes are primarily caused by cardiac autonomic modulation.
The calculation of HR and its variability can be used to estimate autonomic activity as and in particular, to evaluate autonomic nervous system influences on heart functions. The autonomic nervous system (ANS) comprises all of the efferent nerves through the visceral organs, including the cardiovascular system, the and the peripheral involuntary muscles. The ANS is generally described as a combination of two main systems that balances and interacts; the sympathetic, regulated by adrenergic activities, and the parasympathetic, cholinergically regulated. One of the main nerves controlling the activity of the heart is the fast-acting, parasympathetic Vagus nerve.
It is generally accepted today that HRV measurement is also a valuable took for the determination of the status of the ANS. Changes in vagal activity cause immediate large changes in instantaneous HR, whereas changes in sympathetic activities are associated with more gradual, slow changes.
The measurement of HR and its rhythmicity, HRV, are only used as a diagnostic tool in cardiology. A stable heart rate is a sign that the heart does not respond to external influences, which responses are mainly regulated by the ANS. Such a situation is dangerous for the individual and is considered to be a pathological symptom. Research has indicated that a quantification of HRV, the discrete beat-to-beat variability in the heart, plays an important prognostic role as an indicator of risk associated with a large variety of diseases, behavioral disorders, mortality and also aging, independent of other risk factors.
Depressed, low HRV has been shown to be a powerful predictor of cardiac events after myocardial infarct. It is therefore crucial to establish a measure of HRV and to quantitatively classify the HRVs of different pathological cases, in order to discriminate between healthy HR profiles and those of patients at risk.
The commercially available medical device for detection of HVR is the Holter 24-hour recording and analysis instrument. A Holter instrument monitor continuously records heart patterns from electrodes attached to the patient for a 24-hour period. The Holter recording technique records the ECG on analog magnetic tape, and a Holter scanner analyzes the tape 60 or 120 times to produce a final report. A Holter scanner report may contain statistical calculations of the heart activity and a detailed report of abnormal cardiological events, such as sinus pauses and propped beats. A limited factor of this technology is the long, 24-hour measurement time and the lack of a graphic print-out summarizing the entire examination period as well as a material evaluation of the total measurement.
U.S. Pat. No. 5,682,901 (Kamen) discloses a method and apparatus for measuring autonomic activity of a patient during a short duration. The method utilizes a visual description of the recurrence plot and separates between direct pathological states, according to different patterns. The method, however, suffers from the fact that the figures of the recurrence plots do not allow the inspection of the density of the data points, which varies over the whole contour, but only renders information of the general shape of the plot. The method and apparatus of Kamen include performing a calculation of the correlation dimension in order to quantify the degree of heart failure, but such a calculation necessitates a dimension that is partly bases on a visual, subjective inspection. Such a visual inspection is known to be unreliable.
It is therefore a broad object of the present invention to provide an accurate method and system for measuring HRV over a relatively short period of time of 60 minutes or less.
It is a further object of the present invention to provide a method and a system for measuring HRV, giving results which are more precise and easier to evaluate.
It is a still object of tie present invention to provide a method and system enabling a prognosis of the status of a patient with a history of heart failure or impaired heart function, by quantification of the degree of heart failure or heart function impairment.
It is a yet further object of the present invention to provide a method and a system allowing for the classification of patients with a history of heart failure or impaired heart function into the following three groups, ordered according to the risk of death due to heart failure: (1) patients with a minimal risk of sudden death, comparable to that of healthy individuals; (2) patients with an increased risk of sudden death, and (3) patients with a high risk of sudden death.
In accordance with the present invention, there is therefore provided a system for measuring heart rate variability (HRV) of a patient, comprising recording means for obtaining and recording heartbeat-to-heartbeat intervals for a predetermined period of time; process means for digitizing said intervals, forming a recurrence plot, and assigning a unit mass to each point on the plot representing a measured interval, and calculating the determinant by the expression
Qdet=QxxQyy
wherein:
Qxx is the quadrople moment relative to the X axis of the principal coordinate,
Qyy is the quadrople moment relative to the Y axis of the principal coordinate; and
Qdet is the product to Qxx and Qyy.
The invention further provides a method for measuring the heart rate variability (HRV) of a patient, comprising collecting data of heartbeat-to-heartbeat intervals; determining the intervals during a predetermined period of time; generating a recurrence plot from said determined intervals, and calculating the determinant by the expression
Qdetxe2x88x92QxxQyy
wherein:
Qxx is the quadrupole moment relative to the X axis of the principal coordinate,
Qyy is the quadrupole moment relative to the X axis of the principal coordinate; and
Qdet is the product of Qxx and Qyy,
The invention will now be described in connection with certain preferred embodiments with reference to the following illustrative figures so that it may be more fully understood.
With specific reference now to the figures in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of like preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspect of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice.